Since the late 1970s, waveguides have increasingly supplanted conventional signal lines for transmitting information. Rather than encoding information in electrical signals and transmitting the encoded electrical signals via signal lines, the same information can be encoded in a channel of electromagnetic radiation and transmitted via waveguides, such as optical fibers, ridge waveguides, and photonic crystal waveguides. The term “channel,” also called “optical channel,” refers to electromagnetic radiation transmitted at one wavelength through a waveguide. Transmitting information encoded in channels via waveguides has a number of advantages over transmitting encoded electrical signals via signal lines. First, degradation or loss is much less for channels transmitted via waveguides than for electrical signals transmitted via signal lines. Second, waveguides can be fabricated to support a much higher bandwidth than signal lines. For example, a single Cu or Al wire can only transmit a single electrical signal, while a single optical fiber can be configured to transmit about 100 or more channels. Finally, electromagnetic radiation provides, in general, a much higher transmission rate.
Recently, advances in materials science and semiconductor fabrication techniques have made it possible to fabricate waveguides that can be integrated with electronic devices, such as memory and processors, to form photonic integrated circuits (“PICs”), where the waveguides may be used to transmit information encoded in channels between the electronic devices. PICs are the photonic equivalent of electronic integrated circuits and may be implemented on a small wafer of semiconductor material that forms the base of the electronic devices. Unlike electronic integrated circuits where Si is the primary material, PICs may be composed of a variety of materials. For example, PICs may be composed of a single semiconductor, such as Si on an insulator, or binary and ternary semiconductors, such as InP and AlxGa1-xAs, where x varies from 0 to 1.
In order to effectively implement PICs, passive and active photonic components are needed. Waveguides and attenuators are examples of passive photonic components that can be fabricated using conventional epitaxial and lithographic methods and may be used to direct the propagation of channels between electronic devices. Physicists, engineers, and computer scientists have recognized a need for active photonic components, such as modulators, that can be used to encode information in channels for distribution to other electronic devices.